Fabian Eckert, Teresa C. Fort, Peter K. Schott, and Natalie J. Yang. "Imputing Missing Values in the US Census Bureau's County Business Patterns." NBER Working Paper #26632, 2021

URL: http://fpeckert.me/cbp/

From the data provider's README:

The NAICS long panel records employment at the most detailed industry-county level of aggregation possible. The sum of all observations in this panel represents the national total. For reasons explained in detail in our working paper, it is not always possible to assign employment to a six-digit NAICS code. In these cases, the employment is attributed to a more aggregate code, e.g., 11////.

We note that unlike in the raw CBP data and the text, where we describe codes such as 11//// as a root that contains the sum of all codes that are more detailed than 11////, that is not what codes of this form represent in the long panel. Instead, the roots in this dataset contain the remainder of 11//// employment that we could not assign to a more disaggregated NAICS code.  Users can obtain the total 11//// employment by summing employment over all codes that start with 11.